After many tries I finally mastered them. I sew the borders on and then fold to make the miter before I cut. Lay out flat so you can see the miter - then crease and then mark on wrong side with a marking pen. Then I carefully pin and sew. You can sew with a basting stich first to see if you got it right, then sew for keeps and then cut. Hope this makes sense.
It just take practice.